We are upgrading an existing production network and will be making some major changes, but we need to keep things working in the meantime

At the moment the internal IT equipment is all on VLAN1 (native), I want to move all the internal stuff into a new switch which is going to be behind a sonicwall.
There are access points connected to the old switch that need staff who connect to be on the same lan, these APs also have guest VLAN, cameras etc. So I have created VLAN20 as staff and bridged it to the new switch. Looks like this
Office LAN: PC/servers->LANswitch->SonicWall->Mikrotik->Internet
VLAN20 bridge: StaffPhones(untagged)->AP(20)->OldSwitch(20)->Mikrotik(20)->LANswitch(untagged)->SonicWall->Mikrotik->internet

This works perfectly fine, no problems

However..... Some office equipment is still connected to the old switch. And so while we are putting new access points in and moving some office equipment over to the new switch, we still need them to work

So is there a way to bridge VLAN20 on the old switch, and VLAN1/untagged on the old switch together as well?
Everything I have tried doesn't seem to work. I don't know if my config is correct. Normally with old method I would create VLAN interface on all the physical Ethernet interfaces and just bridge everything together. But I'm trying to do this with new bridge config and problem is I can't add ether3 (which is a bridge port) and bridge it to VLAN20 interface, because ether3 is already a bridge port, can't add it twice. Can I add VLAN20 interface as a bridge port and set it as untagged on VLAN1? That doesn't seem to work

I have tried some other methods and the other problem I have is spanning tree blocks one of the ports (either ether3 or ether3.20) I think this is because it's bit Per-VLAN spanning tree. Do ubiquiti and Mikrotik support this to prevent spanning tree thinking this is a loop?

You still can do it "the old way".

Or you can create vlan interfaces for ALL vlans (including the "native" vlan) on top of one bridge, and then bridge the ones you want together.

Ok so I figured it out

There's 2 problems to tackle, the first is the bridge config and that is a combination of the new method and old method. Create a bridge, add all the ports, set all the VLAN's etc according to a new method. Except in my case VLAN20 I don't add under the 'VLAN' section of the bridge. Instead I create VLAN interfaces on each physical interface (in my case only ether3.20 is necessary)

So my Bridge Ports section has all the physical interfaces as per the new method. But then I also go and add the 'ether3.20' interface and set its PVID to 1 (I want to bridge VLAN20 and 1/Native, use the PVID of where its bridging to).
Then in the VLANs section of Bridge, open up VLAN1 and add ether3.20 as an untagged port
NOTE: This will cause a loop if using STP/RSTP and either block off a section of the network, or cause an actual broadcast storm. Disable the port if necessary until the next step is done

The next problem is the loop issue. STP or RSTP is not sufficient because they aren't VLAN aware and will just block a port. So I had to setup MSTP on all the switches, as well as MSTP instances (MSTIs), 1 for VLAN1 and another for VLAN20. I set this router as the root bridge, matched up revision number and region name on all the switches and this router so they all saw it as the root for each VLAN and then it all works as expected. VLAN20 traffic goes to the new switch, and it also goes to VLAN1 on the old switch